<script src="vue.js">

</script>
<body>
    <div id="app">
      <input type="text" v-model="cityName"/>
    </div>

        
    </body>
    <script>
        var vue=new Vue({
            el: '#app',
            data: {
                cityName:"天津"
            },
            watch:{
                cityName(newName,oldName){
                    console.log(newName,oldName);
                    
                }
            
                
            }
        });
    </script>
